Add user action event for expand/collapse on a treeitem

[jnurthen]

Most trees support expand and select as separate actions on tree items.

Windows Narrator allows a user to double tap to expand an item in a tree and triple tap to select it. There is no way to implement this action in a web app.
It would be great for the expand event to fire the right arrow (not sure how trees are represented in LTR so not sure if this would flip) and fire space for the select event. Alternatively click could work for the select but I'm unsure if we want to mix keyboard and mouse events in this way on the same object.

[cookiecrook]

Also multi-select #167

[cookiecrook]

discussion yesterday concluded that LR arrows for expand/collapse was reasonable (with the dir="rtl" caveat). Since select is already implemented as click in most browsers, consensus was that no change to select is necessary.